Chemical vapor deposition (CVD) is a process by which a solid material is deposited from gaseous precursors into a heated substrate. Within the past two decades, CVD processing has been directed into a very successful industry for the fabrication of microelectronic devices. Moreover, the use of CVD processing for the fabrication of high performance cutting tools and high temperature coatings is expanding rapidly.
A recent review on advanced ceramics by CVD has summarized the significant developments in this field (David P. Stinton, Theodore M. Bessman and Richard A. Lawden, "Advanced Ceramics by Chemical Vapor Deposition Techniques", Amer. Cer. Soc. Bull., 67, (1988), 350, and references therein). These developments fall into three categories as follows.
(1) surface modification by CVD, PA1 (2) composite fabrication by chemical vapor infiltration (CVI) and PA1 3) CVD formation of thin films for electronics applications. PA1 exposing a pre-formed metal substrate to a NaCl filtered continuous wave laser beam of about 50 to 100 W power and about 80 to 315 W/cm.sup.2 power density in an atmosphere of a gas desired to react with the metal; PA1 allowing for the gas to diffuse into the metal substrate at a temperature effective to permit reaction thereof to form the ceramic monolith; and PA1 cooling the monolith. PA1 exposing a pre-formed metal substrate to a NaCl filtered continuous wave laser beam of about 50 to 100 W power and about 80 to 315 W/cm.sup.2 power density in an atmosphere of a gas desired to react with the metal; PA1 allowing for the gas to diffuse into the metal substrate at a temperature effective to permit reaction thereof to form the ceramic monolith; and PA1 cooling the monolith.
Examples of related prior art encompass the following patents.
U.S. Pat. No. 3,379,555 to Hough describes the coating of a substrate such as tungsten with graphite. This is done by heating a filament of tungsten to 3500.degree. C. and contacting the filament with a gas mixture of argon and an amine of an alkyl hydrocarbon such as methane.
U.S. Pat. No. 4,016,013 to Bitzer et al discloses the use of heat or radiant energy for depositing the fusing layers of carbides or nitrides on a metal or metalloid such as silicon or molybdenum. Compounds like triazines or pyrimidines are made to react with the substrate. Additives such as methane and ammonia as sources of carbon and nitrogen are also described. The substrate can be in the form of a powder or a machined article and the reactor is described as being of the CVD type.
U.S. Pat. No. 4,248,909 to Whittaker describes the preparation of an optical coating where a laser beam is focused on a graphite rod to produce gaseous carbon. The substrate can be a metal or a non-metallic crystalline solid. Upon impingement on the substrate surface the carbon gas is quenched to form a carbine film.
U.S. Pat. No. 4,434,189 to Zaplatynsky describes the coating by alloying or forming TiN on the metal substrate surface, preferably of titanium and titanium alloys. This is done by nitriding. A laser beam of the carbon dioxide type is passed through a sodium chloride window to strike the substrate, thereby causing rapid heating. The heated substrate reacts with nitrogen which forms first a solid solution and then titanium nitride. It is stated that the process may be used in the formation of ceramic coatings on ceramic materials.
U.S. Pat. No. 4,552,786 teaches the use of a supercritical fluid such as carbon dioxide to carry ceramic precursors such as polycarbosilane polymers into the pores of a ceramic substrate. Materials such as aluminum borosilicate are exemplified as the ceramic substrate (see, Example 4, Col. 4 of the patent). The polysilane is dissolved in this supercritical fluid and then infiltrated into the borosilicate. Moreover, it is only a small area of the substrate surface which is treated at any one time and the power density of the laser ray which is utilized is at least 20,000 W/cm.sup.2, substantially higher than in the inventive method.
U.S. Pat. No. 4,574,459 describes the preparation of extrusion dies suitable for fabrication of ceramic monoliths where steel is coated with a compound such a titanium carbide by the CVD process.
U.S. Pat. No. 4,569,855 to Matsuda et al describes the deposition of a film such as a semiconductor film on a metal substrate such as molybdenum (see Col. 5, lines 40-42 of Matsuda et al). A silicon compound having azo or azide groups is excited by a laser beam and decomposed to form a gas. A uniform deposition film is formed. This method is known as a photo-CVD method.
U.S. Pat. No. 4,822,751 to Ishizu et al provides a method for forming a thin film semiconductor device by producing a patterned metal film on a substrate and depositing by CVD. The patterned film is placed in a SiH.sub.4 atmosphere and illuminated with an argon laser beam which gemrates heat to decompose the SiH.sub.4 gas beam which generates heat to decompose the SiH.sub.4 gas giving silicon, which is then deposited on the thin film in an aligned manner.
U.S. Pat. No. 4,810,580 describes the nitridation of silicon atoms in clay by reaction of the clay with ammonia at temperatures above 1000.degree. C. The result is the formation of silicon nitride groups and the product is said to be useful in ceramics.
U.S. Pat. No. 4,828,874 to Hiraoka et al describes surface treatment of polycrystalline silicon by chlorine gas particles excited with a laser light. The treatment is said to be for the manufacture of surface elements.
U.S. Pat. No. 3,947,653 to Fairbairn discloses a surface treatment for metallic articles which comprises forming a eutectic surface on a metal article by means of high density energy and heating to a temperature between the surface melting temperature and a surface vaporization temperature for a short period of time to melt the surface layer and cooling at a specified rate.
U.S. Pat. No. 4,157,923 to Yen et al. describes a method for increasing the physical properties of nonallotrophic metals by means of a high energy beam of at least 10,000 W/cm.sup.2 to produce a rapid self-quenching rate and produce a precipitate and/or intermetallic compound in the resolidification zone.
U.S. Pat. No. 4,212,900 to Serlin describes a method for allowing the surface of a substrate by directing a beam of high intensity energy and alloying material placed on the surface of the substrate.